We present Gran Telescopio Canarias (GTC) optical transit narrowband photometry of the
hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This unique instrument has the
capabilities to deliver high-cadence narrowband photometric lightcurves, allowing us to
probe the atmospheric composition of hot Jupiters from the ground. The observations were
taken during three transit events that cover four wavelengths at spectral resolutions
near 500, necessary for observing atmospheric features, and have near-photon limited
sub-mmag precisions. Precision narrowband photometry on a large aperture telescope allows
for atmospheric transmission spectral features to be observed for exoplanets around much
fainter stars than those of the well studied targets HD 209458b and HD 189733b, providing
access to the majority of known transiting planets. For XO-2b, we measure planet-to-star
radius contrasts of
Rpl/R⋆ = 0.10508 ± 0.00052
at 6792 Å, 0.10640 ± 0.00058 at 7582 Å, and 0.10686 ± 0.00060 at 7664.9 Å, and
0.10362 ± 0.00051 at 8839 Å. These measurements reveal significant spectral features at
two wavelengths, with an absorption level of 0.067 ± 0.016% at 7664.9 Å caused by
atmospheric potassium in the line core (a 4.1-σ significance level), and
an absorption level of 0.058 ± 0.016% at 7582 Å, (a 3.6-σ significance
level). When comparing our measurements to hot-Jupiter atmospheric models, we find good
agreement with models that are dominated in the optical by alkali metals. This is the
first evidence for potassium in an extrasolar planet, an element that has along with
sodium long been supposed to be a dominant source of opacity at optical wavelengths for
hot Jupiters.

Based on observations made with the Gran Telescopio Canarias (GTC), installed in the
Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de
Canarias, in the island of La Palma, and part of the large ESO program 182.C-2018.

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